EXTENSIVE DENITRIFICATION AND NITRATE LIMITATION IN AN EARLY TONIAN MARINE BASIN

Connecting the “Boring Billion” and Cryogenian “Snowball Earth” events, the Tonian Period marks a critical transition in Earth history. Recent paleobiological studies have shown that major eukaryotic evolutionary divergences and innovations occurred in the Tonian Period. These events may have been driven by the transition from sulfidic to ferruginous mid-depth seawaters during the Tonian, which possibly created more favorable environmental conditions for the diversification of eukaryotes. To improve our understanding of Tonian oceanic redox conditions, we carried out a systematic investigation of shallow-water carbonates of the Mesoproterozoic–Tonian Huaibei Group in North China. New rare earth element and iron speciation data from the Huaibei carbonates suggest that ferruginous conditions possibly expanded into shallow waters, which would have had an impact on the nitrogen and phosphorus cycles within the basin. It has been suggested that widespread ferruginous conditions may lead to phosphorus limitation due to a trapping of phosphorus on Fe-oxide minerals within the sediment. Nitrogen isotope composition of bulk carbonate samples (δ¹⁵N_bulk) from the Huaibei Group show near-zero δ¹⁵N_bulk values in the lower part of the Huaibei Group and positive δ¹⁵N_bulk values (> 2‰) in the upper part. Near-zero values of the lower Huaibei Group are interpreted as evidence for extensive denitrification and quantitative removal of nitrate from ferruginous shallow seawaters, leading to an ammonia-dominant ecosystem. In contrast, positive δ¹⁵N_bulk values in the upper Huaibei Group suggest partial denitrification as shallow seawaters became more oxygenated during the later Tonian. Since eukaryotes can outcompete prokaryotes when nitrate is sufficient, the increasing nitrate availability and seawater oxygenation of shallow marine environments, as evidenced by geochemical data from the Huaibei Group, may have played an important role in facilitating eukaryote diversification in the Tonian Period.